Israeli warplanes carried out a strike deep inside Syrian territory on Wednesday, U.S. officials reported, saying they believed the target was a convoy carrying sophisticated anti-aircraft weaponry on the outskirts of Damascus that was intended for the Hezbollah in Lebanon.

The U.S. officials, who spoke on the condition of anonymity, said Israel had notified the United States about the attack, which the Syrian government condemned as an act of “arrogance and aggression”. Israel’s move demonstrated its determination to ensure that Hezbollah — its arch foe in the north — is unable to take advantage of the chaos in Syria to bolster its arsenal.

The predawn strike was the first time in more than five years that Israel’s air force had attacked a target in Syria. While there was no expectation that the beleaguered Assad government had an interest in retaliating, the strike raised concerns that the Syrian civil war had continued to spread beyond its border.

In a statement, the Syrian military denied that a convoy had been struck. It said the attack had hit a scientific research facility in the Damascus suburbs that was used to improve Syria’s defences, and called the attack “a flagrant breach of Syrian sovereignty and airspace”.

One U.S. official said the trucks targeted on Wednesday were believed to have been carrying sophisticated SA-17 anti-aircraft weapons.

Israeli officials would not confirm the airstrike, a common tactic here. But it came after days of intense security consultations with Prime Minister Benjamin Netanyahu regarding the possible movement of chemical and other weapons around Syria, and warnings that Israel would take action to thwart any possible transfers to Hezbollah.

Thousands of Israelis have crowded gas-mask distribution centres over the last two days. On Sunday, Israel deployed its Iron Dome missile defence system in the north, near Haifa, which was heavily bombed during the 2006 war with Lebanon.

Syria and Israel are technically in a state of war but have long maintained an uneasy peace along their decades-old armistice line.